Iron deficiency is probably the most widespread nutritional problem in industrialized nations, yet there is a paucity of literature on the effects of iron deficiency on the ageing process. Because the population is continually exposed to chemicals in the environment and increasing quantities of therapeutic drugs are being consumed during the ageing process, it is imperative that studies be conducted to determine the interrelationship of diet and the ageing process on the drug-metabolizing system. We are approaching this problem by measuring the activities of specific enzymes associated with the mixed-function oxidase system in the microsomes of the liver and kidneys of rats at various ages. Four experimental groups are being used: (1) iron-replete males, (2) iron-deficient males, (3) iron-replete females, and (4) iron-deficient females. The enzyme activities being monitored are (1) cytochrome P-450, (2) heme oxygenase, (3) p-chloro N-methylaniline N-demethylase, (4) aniline hydroxylase, (5) benzopyrene hydroxylase, and (6) microsomal NADPH cytochrome C reductase. In addition, the effects of stannous chloride induction of heme oxygenase synthesis on the enzymes are being assayed. Heme oxygenase activity was stimulated twofold to threefold in the liver and fivefold to tenfold in the kidneys after stannous chloride injection. Although liver P-450 levels in males was about twice that in females, these levels were depressed 30-50% in both groups after stannous chloride injection. Benzypyrene hydroxylase activity was 9 to 10 times higher in the males than in the females. The enzymatic activities of cytochrome C reductase, p-chloro N-methylaniline N-demethylase, and aniline hydroxylase appear to parallel liver iron concentrations in that with reference to 6-month-old animals the values appear to increase at 12-months of age and to decrease at 18-months of age.